Such arc gears are used, for example, in the cyclo gear known to the specialists, for the 1:1 transmission (coupling) of the torque and the speed of the shaft rotating about its own axis eccentrically (or with a distance between centers) with respect to other shafts. The radius of the concave surfaces is preferably greater than that of the convex surfaces by a factor corresponding to the eccentricity (the distance between centers). Such a gear generally avoids the long design of a cardan shaft, which in fact is known to perform the same task in the case of shafts arranged virtually parallel. Such gears have recently also been used in hydrostatic rotary piston machines having a large intake volume, based on the so-called "orbit" principle for preventing rotating of one of the two elements of the displacement gear, which elements execute an eccentric movement, this being the case in particular when a so-called Eaton tooth system is provided.
"Eaton tooth system," as explained in European Patent 43899 refers to a gear system of an annular gear pump having a housing, an internally toothed annular gear-wheel with eight to sixteen teeth, rotatably mounted in the housing, and a pinion carried by a drive shaft. The pinion has one tooth less than the annular gear-wheel and meshes with the annular gear-wheel. The sealing between suction chamber and pressure chamber is effected opposite the point of deepest engagement of the gear-wheel by sliding of the tips of the teeth of the pinion on the teeth of the annular gear-wheel and at the point of deepest engagement of the gear-wheel by bearing of the driving flanks of the teeth of the pinion against the teeth of the annular gearwheel. Furthermore, the tips of the teeth of the pinion go freely in the gaps between the teeth of the annular gearwheel. The tooth profile of the pinion is defined by rolling thereof in the annular gear-wheel.
As mentioned above, an important advantage of such arc gears is their extremely short design and their ability, in the case of staggered shafts, to transmit the speed at each angle of rotation in an exactly constant manner from one shaft to another. However, the problem in the known embodiments is the large arc diameter required for the power transmission surfaces and the high Hertzian stresses at the points of engagement during transmission of high torques. Since, furthermore, the active engagement region of the power transmission surfaces is relatively small owing to the system, the required installation space is in the end very large compared with the benefit. This is also evident, for example, in the case of the gear according to German Offenlegungsschrift 2,007,853. To achieve a sufficiently high contact ratio there, the number of teeth must be increased. However, this automatically has the disadvantage once again that the diameter of the overall gear has to be increased and hence its size. In some gears, however, such a size increase is not acceptable, and it has therefore been necessary in such cases once again to rely on cardan solutions.
The term "cardan solution" refers to a gear with a universal or cardan joint, or a propeller shaft with universal joints, or a cardan shaft.